The present invention relates to a method of mounting a target plate to be cooled into a vacuum process chamber; an arrangement of a target plate to be cooled, and a vacuum process chamber, wherein said target plate is to be installed, the present invention also relates to a target plate for a vacuum coating process and a vacuum chamber for vacuum-process-coating workpieces by target sputtering and target evaporation.
It is well-known, that various vacuum processes use target plates, which are consumed during the process. In reactive or non-reactive coating processes, the target surface is sputtered by applying a DC and/or an AC glow discharge, simultaneously, along a relatively large surface area, and material particles are deposited on workpieces, either directly or after a chemical reaction in which a reactive gas is introduced into the vacuum chamber.
Thus the target is often operated within a magnetron arrangement for so-called high rate sputtering, by which magnetronically the density of ions adjacent to the target surface is increased.
In other coating processes, the target is evaporated by an electric arc or by the action of an electron beam.
In each of these cases, the target is highly loaded thermically by the sputtering or evaporation process, either through ion bombardment, electron bombardment or by the electric arc drawn onto the target surface.
All target plates should be replaceable in a simple manner and without disassembling the parts of the vacuum chamber. It must often be ensured that no material of the mounting members is exposed to the process chamber, which is different than the material sputtered or evaporated, because such additional material could ruin the delicate coating processes.
In order to remove the heat resulting from these processes, the target plates are usually mounted on a cooling plate arrangement.
The largest amount of heat can be removed from such target plates when these plates are in direct contact with a cooling medium in liquid or gaseous form. Such a cooling approach is hardly used due to the sealing considerations with respect to the vacuum process chamber.
Thus, it is known, e.g. from the European specification EP-A-0 334 347, or from the article "Cathode Cooling Apparatus for a Planar Magnetron Sputtering System", M. R. Lake and G. L. Harding, J.Vac.Sci.Technol. A 2(3), July/September 1984, American Vacuum Society, p. 1391 ff., or from the Sovjet Patent Specification No. 823 459, the EP-A-0 393 344, to separate a conduit system for the cooling medium from the inside surface of the target plate by a thin membrane-like or foil-like heat conducting partition wall, whereby at the one side, the vacuum technical sealing problems are resolved with respect to a direct contact cooling. On the other side, due to the pressure of the cooling medium onto the membrane-like partition wall and therefrom onto the inside surface of the target plate, there is realized a very accurate heat transition from the target plate to the cooling medium. Thereby, the target plate withstands the forces which are generated by the pressure of the cooling medium. If the mechanical loading capability of the target plate is too low, for instance due to its dimensioning and/or to the characteristics of the target material, it is common to provide a target plate made from a plate of target material and from a target support plate, in which the plates are bonded together. Thus, the supporting plate takes up the major part of the mechanical loading of the target plate.
Where small target plates are used, the small target plates are mounted at their periphery, e.g. by means of a clamping frame. Larger target plates are commonly clamped in their central area.
The mounting of the target plate is usually performed either from the side pointing towards the area within the process chamber wherein the process occurs, i.e. from the front side, or from the side pointing towards the chamber wall of the process chamber, i.e. the cooling plate, from the rear. A mounting of the target plate from the front (according to the Swiss specification no. CH-A-644 303) has the advantage that the target plate may easily be removed and mounted without the need of having to disassemble the cooling plate. However, a disadvantage is that the material of the mounting means, such as as the material of screw bolts, and differs from the material of the target plate, and is exposed to the process area within the process chamber, which disables due to sputtering or evaporating the entire target surface. Thus, the areas of this surface adjacent to other material mounting means may not be exploited without ruining the coating process by the material of the mounting means. This has an impact with regard to the distribution of the coating thickness of the layer resulting from sputtering. This limits the exploitation of the target plate which is often made of expensive material.
A co-sputtering of such mounting means or members arranged at the front side of the target plate, i.e. the side exposed to the sputtering process or to the evaporation process, may never be completely excluded. Therefore, such a target plate mount is avoided for delicate coating processes which must meet the highest demands.
A target mount with screw bolts introduced from the rear side of the target plate is taught by specification no. CH-A-669 242 which has the advantage that the target plate may be sputtered or evaporated along its entire front surface, but has the disadvantage that when replacing the target plate the cooling plate must be disassembled in order to provide access to the mounting bolts.
It is taught by U.S. Pat. No. 5,009,765 to bond or weld a target plate onto a supporting cylinder. The supporting cylinder comprises radially projecting lugs at a radially projecting flange with which the supporting cylinder is mounted in recesses provided at the wall of the vacuum process chamber. The supporting cylinder with the target plate welded thereto defines an interior chamber which is substantially higher than the thickness of the target plate. This volume is relatively large so as to allow it to be filled with an relatively large volume of a cooling liquid.
However approach has disadvantages in that, together with the target plate, a relatively complicated additional part, namely the supporting cylinder, must be exchanged. This supporting cylinder is in general not made of the same material as the target plate due to the machining requirements and/or due to cost considerations. In the case where the target plate and the supporting cylinder are made of different materials, the material of the supporting cylinder being co-sputtered will ruin the coating process.
Also, the mount of the target plate, namely the welding seam, is exposed to high thermical and mechanical (cooling medium pressure/vacuum pressure) loadings, which present, under consideration of different pairs of materials for the target plate and the supporting cylinder, additional problems.
According to EP-A-0 393 344, only the target plate itself is exchanged, but holding frames are provided which are exposed to the process and are thus co-sputtered and against which the pressure of a cooling medium acts via a sealing membrane. The holding frames are generally not made of the same material as the target plate, due to the costs which would arise if these frames were machined from a highly expensive target material or due to machining problems which would arise and which would cause additional costs if such frames would have to be machined from the target material.